Post-ecological effect and risk assessment of using modified clay in harmful algal bloom mitigation: An attempt based on the responses of zooplankton Brachionus plicatilis and bivalve Mytilus edulis

Abstract

The modified clay is the only worldwide-accepted practical method for mitigating algal bloom. Is it ecologically safe? To evidence it, a simulative bloom-occurring system of Karenia mikimotoi was set up, and the sentinel organisms of rotifer Brachionus plicatilis in sea surface and blue mussel Mytilus edulis on the benthos were respectively included. The organisms' physiological responses were determined as the indicators to reflect the ecological impacts when clay settled from surface to the bottom during the mimic bloom-mitigating process. Modified clay at a concentration of 0.1 g/L effectively removed the K. mikimotoi at an 81% removal rate, and its addition would not significantly strengthen the negative impacts on population dynamics and reproductive activities of B. plicatilis induced by sole K. mikimotoi within the first 2 h. Even an alleviation was observed at 2 d indicated by the increase of survival rate, egg and larva production after clay addition compared with those of 2 h. When the clay particles settled to benthos, the physical damage to the gills and digestive glands of M. edulis were found via the tissue and SEM observation, especially in higher treatment groups of 0.5 and 1.0 g/L, and filtering rate, digestive enzymes, condition index, water content and mortality were also influenced. However, little impact was found in group of 0.1 g/L. Risk assessment based on the adverse outcome pathway (AOP) model further revealed that the complete key event-key event relationship-adverse outcome pathway was only clearly observed in 0.5 g/L and 1 g/L groups but not in 0.1 g/L group, inferring the small ecological risk of 0.1 g/L. The integrated biomarker response (IBR) based on the mussel's physiological responses further backed up the AOP outcoming. The combined results from rotifer to bivalve emphasized on one conclusion that modified clay at 0.1 g/L was effective and ecologically safe in coastal bloom mitigation.

Keywords: Algal bloom mitigation; Modified clay; Post-ecological effect; Risk assessment; Zooplankton Bivalve.